The advancement of digital image sensing apparatus such as a digital video camera has been very rapid these days. The number of pixels and density of pixels have been improved, and a digital image sensing apparatus capable of sensing an image with the quality substantially equivalent to the quality of an image sensed by a conventional image sensing apparatus using a silver halide film. In such digital image sensing apparatus in general, analog signals obtained by photoelectrical conversion are read out from an area sensor pixel by pixel and undergo analog-digital (A/D) conversion to form an image using the digital data. As the number of the pixels increases, the resolution of an image as well as the quality of the image improve. However, if the processing speed of A/D conversion does not improve, the time taken to read a frame of an image is prolonged, which makes the frame rate—the number of images that can be sensed per second—decrease. Accordingly, there are desires to achieve the following: increase a reading speed of an area sensor and an A/D conversion speed, and improve a multi-channel reading technique of reading a frame image via a plurality of channels.
In a method of increasing the reading speed and A/D conversion speed, there are problems in the response speed of an amplifier used for reading, as well as clock noise. These problems constrict a good phase range for an A/D conversion period with respect to a read-out period. The same problems also cause phase shift, and make noise conspicuous in an image, and/or cause pattern noise. Accordingly, it was very difficult to achieve an image of high quality. As for multi-channel reading, noise generated in multiplexing signals read via a plurality of channels and cross-stroke between channels also constrict a good phase range for an A/D conversion period with respect to a read-out period, which also makes it difficult to achieve an image of high quality. In addition, pulses used for reading pixel signals from an area sensor are generated by a timing generator (TG), and a delay since the pulses are generated until the signals are actually read out changes depending upon temperature. The delay also differs from one image sensor from another. Thus, it was very difficult to compensate a phase shift using a circuit so as to always perform A/D conversion with a good phase to output read signals.